st/mesa: always report the max samples as supported

This is required by GL. Doing this even when the backend driver does not support it leads to creating attachments which are not renderable. This is not ideal, but does pass tests. This covers scenarios such as missing MSAA for 128-bit formats on Sandybridge, and missing 8x MSAA on 128-bit formats on NVIDIA Tesla generation boards.

Signed-off-by: Ilia Mirkin imirkin@alum.mit.edu